The present invention relates to the D.C. brushless motor and its driving control system, and, in particular, to the D.C. brushless motor and its driving control system which are suitable for use as the driving source of a magnetic disc driving system that drives a disc-shaped recording medium for the write-in and read-out of information through relative motion against a detector.
Prior art is described hereinafter along FIGS. 1 through 5. FIG. 1 is to help explain one example of conventional magnetic disc driving system, and (a) exhibits its top view while (b) shows its side view. In the FIG. 1, 101 is the base board which is fixed to the outer case 108 covering the whole device; 102 is the bearing part which holds the axle of the magnetic disc 107; 103 is an electric motor for driving the system, while 104 and 105 are the pulleys respectively and 106 is the belt. The revolution of the driving motor 103 is transmitted to the magnetic disc 107 by way of the pulley 104, belt 106 and pulley 105, and drives the magnetic disc 107 to rotate at a certain set revolution speed, whereby information is written in to or read out from the magnetic disc 107 by means of the magnetic head (not shown in the drawing) which is installed close to the magnetic disc 107.
The function of the driving motor 103 as shown in FIG. 1 is to rotate the magnetic disc at a certain set revolution speed, but conventionally in most cases induction motors have been employed as the driving motor 103. The conventional driving system, however, which drives the machine by the induction motor, pulley and belt, involved a number of problematic points. To illustrate some of the problems, the conventional driving system requires an adjustment in the revolution speed of magnetic disc to fit the frequency of the power source employed by changing the pulley's diameter according to the frequency as the synchronous speed of induction motor varies according to the frequency of the power source employed. The conventional driving system further requires special design to meet such complex forces as the strong side pressure and axlewise load which the bearings of the motor axle and the magnetic disc axle receive due to the strong tension added to the belt in order to prevent slipping between the pulleys and the belt. The conventional system also generates a large loss of power by the strong set-in pressure and side pressure added to the transmission loss of the belt thus requiring an induction motor with a far greater capacity than its actual net power. The conventional system also requires much longer time from start of the magnetic disc's rotating movement to reach a stable rotating speed as it drives the magnetic disc which has a rather large inertia moment whereby the loss of time and power it suffers is rather substantial. Furthermore, as the conventional system employs a driving system dependent upon pulleys and a belt, the interior of the outer case is contaminated by flying dust generated from the worn-out belt, and it necessitates such designings as to shut off the air flow between the driving compartment and the magnetic disc compartment, while in addition it requires such upkeep as frequent replacement of belt and cleaning of accumulated dust, which detract its operating efficiency. As such, the conventional induction motor driving system embraced numerous problems.
In order to cope with these problems, conventionally a procedure has been employed, wherein an motor directly drives the magnetic disc or recording cylinder by way of using the same axle for the rotating axle of the magnetic disc or recording cylinder and simultaneously for the driving axle of the D.C. brushless motor without using any pulley-and-belt transmission system. FIG. 2 shows one example of the driving circuit of the D.C. brushless motor which is conventionally employed for driving the turntable of record players. In FIG. 2, 110 is the rotor magnet and 111 and 112 are stator coils respectively which are placed in respective positions opposing the rotor magnet 110 across air gap while the stator coils 111 and 112 are so arranged in their respective positions as to be at 90 radian to each other in the electric angle. Elements 113 and 114 are magnetic induction elements respectively, e.g. Hall elements, which are placed close to the stator coils 111 and 112, the magnetic induction element 113 being placed in a position where an output voltage of almost sine-wave to the rotating angle of the rotor 110 is made available almost at the center of the stator coil 111 of the first phase, and it forms the circuit connection which facilitates the amplification of the output voltage of this magnetic induction element 113 by the amplifier 120 before the current flows to the stator coil 111. Similarly, the magnetic induction element 114 is placed in a position where an output voltage of almost cosine-wave to the rotating angle of the rotor 110 is made available almost at the center of the stator coil 112 of the second phase, and it forms a circuit connection which facilitates the amplification of the output voltage of this magnetic induction element 114 by the amplifier 130 before flowing to the stator coil 112. Furthermore, in the example shown in FIG. 2, the amplifier 120 is composed of an operating amplifier 121, four transistors 122, 123, 124 and 125 and four resistors 126, 127, 128 and 129, while the amplifier 130 is composed of an operating amplifier 131, four transistors 132, 133, 134 and 135 and four resistors 136, 137, 138 and 139.
FIG. 3 is the chart which illustrates the relations of the rotating angle of the rotor 110 of a brushless motor with the current i (111) which flows through the stator coil 111 of the first phase and the current i (112) which flows through the stator coil 112 of the second phase, and between the currents i (111) and i (112) there is a phase difference of 90 degrees. The output torque in this case is expressed as T.sub.1 =K.multidot.sin.sup.2 .theta., T.sub.2 =K.multidot.cos.sup.2 .theta., T.sub.0 =K.multidot.(sin.sup.2 .theta.+cos.sup.2 .theta.)=K, where .theta. is the rotating angle of the rotor 110, T.sub.1 is the torque generated by the stator coil of the first phase, T.sub.2 is the torque generated by the stator coil of the second phase, T.sub.0 is the compound torque and K as the constant, and the torque becomes constant irrespective of the rotating angle .theta. of the rotor 110, so that, if it is used for driving the turntable of a record player, it facilitates minimizing uneven rotation.
The conventional example as shown in FIG. 2, however, was not free from the following problematic points. Namely, in order to make the conventional example exhibited in FIG. 2 absolutely free from any uneven rotation and to sustain the operation with a constant torque, it is imperative that the respective outputs of the magnetic induction elements 113 and 114 should be entirely, in a opposite direction to each other, and the two stator coils 111 and 112 should be in a perfectly identical shape while locating their respective positions at the phase difference of exactly 90 degrees, and that the relative positions between the stator coils and the magnetic induction elements should be kept precise and correct. In the practical version of such electric motors, however, the aforesaid various features and incorrect relative position are in reality being supplemented or corrected by various adjusting contrivances in order to minimize the unevenness in rotation.
The point which bears a specific importance among them is the fact that the characteristic feature of the output of the magnetic induction elements in the plus direction differs from that in the minus direction and to locate these elements in their accurately correct positions is very difficult, and the inconveniencies accruing therefrom generate a difference in torque in the two directions thereby causing a widely uneven torque. In order to supplement this, adjustment can only be made by checking the characteristic feature of each magnetic induction element, which is the one reason impeding their mass-production.
Furthermore, in the structure of the conventional example in FIG. 2, as it is necessitated to flow a sine-wave current through the stator coils, the amplifiers 120 and 130 have to be operative in a linear range, which raise the problem of current efficiency. For the improvement of the efficiency, expanding the operating range of the amplifiers from the linear range to the saturating range works out, i.e. to convert it to the switch-drive system, but conversion to the switch-drive system with the structure as per FIG. 2 remaining unchanged brings about an inconvenience of causing a big vibration noise due to an amplified torque fluctuation.
To cope with this problem, conventionally suggested was a three-phase driving circuit, which puts together three sets of a single-phase driving system, with each of them is composed of one each of magnetic induction element, amplifier and stator coil, and the alignment pitch of the coil of each phase is set at 120.degree. to each other. FIG. 4 is the drawing showing one example of this kind of conventional circuit. In FIG. 4, 151 is the stator coil of the first phase, 152 is the stator coil of the second phase and 153 is the stator coil of the third phase; 161, 162 and 163 respectively are the magnetic induction elements arranged correspondingly to the stator coils 151, 152 and 153; A.sub.1, A.sub.2 and A.sub.3 respectively are the amplifiers which amplify the output of the respective magnetic induction elements; 170 is a 120.degree. logic circuit; 180 is the velocity generator which detects the frequency signals proportionate to the rotating speed of the rotor of the D.C. brushless motor; 190 is the frequency-voltage converter which converts the output frequency signals of the velocity generator 180 into direct current voltage signals; 200 is the current control circuit that puts out currents i (151), i (152) and i (153) which flow to the stator coil of each phase 151, 152 and 153 upon receiving the output signals from the 120.degree. logic circuit 170 as input and the frequency-voltage converter 190; 201 through 206 is the group of transistors amplifying the output of the current control circuit 200; V.sub.H is the Hall bias source when a Hall element is employed as a magnetic induction element, while Vcc is the driving voltage source of the transistor group 201 through 206. The circuit as per FIG. 4 is so structured as to detect the magnetic field of the rotor by positioning the magnetic induction elements 161, 162 and 163 close to the stator coils 151, 152 and 153 respectively and feed the relative input to the 120.degree. logic circuit by way of amplifiers A.sub.1, A.sub.2 and A.sub.3, the output of which then is fed to the three-phase stator coils 151, 152 and 153 after being amplified by the three-phase bipolar driven amplifier which is composed of the transistors 201 through 206. FIG. 5 exhibits the relations, in a single phase, of the current flowing through the magnetic induction elements and the stator coil of each phase and torque with the rotating angle of the rotor in the circuit demonstrated in FIG. 4.
The difference between the structures of the conventional example in FIG. 2 and that of the conventional example in FIG. 4 are in part that the former is a two phase motor while the latter motor has three phases. Therefore, the structure of FIG. 4 is provided with a 120.degree. logic circuit which controls current flow in relation to rotor position as the stator coil is fed with current every 120.degree. derived from the 180.degree. output of the magnetic induction elements both on the plus side and the minus side. In the structure of FIG. 2 on the other hand the outputs of the magnetic induction element on the plus and minus sides spanning 180.degree. are amplified as is and are fed to the stator coil. In order to start feeding current which continues its flow for the period of 120.degree. from the start-up point of the output signals from the magnetic induction element, the magnetic induction element is arranged in the position where its phase is advanced by .pi./6 radian or .pi./6+n.pi. radian in the electric angle from the center of its corresponding stator coil. As shown in FIG. 5, as against the magnetic field which is generally magnetized in a block-type, the coil current flows at the almost flat center 120.degree. part so that the torque is expressed in a flat value as the product of the magnetic field and the current. FIG. 5 exhibits this only for one single phase, but the torque compounding the torques of these three phases also has a lesser torque ripple. Even if there exists a slight discrepancy in the positioning of each magnetic induction element or coil, it only necessitates a slight move-up or -down of the switching point of the current and it does not produce any reverse torque, so that it presents no possibility of producing any large torque ripple. The system of the conventional example shown in FIG. 4, however, performs a 120.degree. flow of current, so it required a 120.degree. logic circuit and had a problem of making its circuit complicated.